In a triple quadrupole mass spectrometer (TQMS) or ion trap time-of-flight mass spectrometer (IT-TOFMS), ions having a specified mass-charge ratio m/z are selected as precursor ions from among the ions derived from the component to be analyzed, those precursor ions are dissociated through collision induced dissociation (CID), and the product ions produced thereby are subjected to mass spectrometry, allowing an MS/MS (=MS2) spectrum to be generated. Furthermore, in an IT-TOFMS, by repeating ion selection and CID multiple times and performing mass spectrometry on the finally generated product ions, it is possible to generate an MSn spectrum, where n is 3 or greater. In the present specification, a mass spectrometer capable of MSn analysis where n is 2 or greater will be referred to as an MSn mass spectrometer.
In a chromatograph-mass spectrometer combining a liquid chromatograph (LC) or gas chromatograph (GC) with the above-described MSn mass spectrometer, if a component contained in the sample is already known, it is possible to preset the mass-charge ratio of precursor ions which are the object of MSn analysis for the holding time of that component, and to obtain the MSn spectrum of the target component. However, if the component contained in the sample is unknown, it is not possible to preset the precursor ions for performing MSn analysis, nor is it possible to obtain MSn analysis results for unknown components contained in the sample besides the target component. To address this, a device is known in the prior art which comprises a function (referred to hereinafter as auto MSn function) for automatically selecting the appropriate precursor ions based on results obtained through MS1 analysis not involving CID, and performing MSn analysis in real time.
For example, Patent literature 1 describes selecting peaks in sequence starting with the one with the highest signal intensity from among multiple peaks appearing in the mass spectrum obtained through MS1 analysis, automatically selecting the corresponding ion species as the precursor ion, and performing MS2 analysis. Furthermore, the same literature describes selecting peaks whereof the signal intensity is within a predetermined intensity range, automatically selecting the corresponding ion species as a precursor ion, and performing MS2 analysis. Furthermore, Patent literature 2 and Non-patent literature 1 describe performing filtering, on multiple peaks appearing in a mass spectrum obtained through MS1 analysis, based not just on signal intensity and mass-charge ratio sequence but also on monoisotopic peaks, valence, etc., or excluding and prioritizing specified ions and then automatically selecting precursor ions and performing MS2 analysis.
FIG. 7 schematically explains the auto MSn function in a common chromatograph-mass spectrometer. Here, as the precursor ion selection criteria, the two peaks with the highest signal intensity from among those peaks in the MS spectrum having a signal intensity at or above a threshold th are selected as the precursor ions. However, an excluded ion list and priority ion list are established separately, as shown in FIG. 2, and ions having a mass-charge ratio registered in the excluded ion list are not selected as precursor ions even if they meet the aforementioned criteria, and conversely, ions having a mass-charge ratio registered in the priority ion list are selected as precursor ions whenever they are present, even if they do not meet the aforementioned criteria. Normally, an excluded ion list is used to prevent selection as precursor ions of known impurity components and interfering components contained in the sample and components known in advance not to require analysis. Conversely, a priority ion list is used to ensure that even trace amounts of components which one wishes to analyze will be selected as precursor ions. It will be noted that the number of precursor ions which can be selected for a single MS spectrum is limited due to time restrictions for performing MSn analysis in real time.
It will be assumed that the MS spectrum shown in A is obtained at time t1 when the waveform of a total ion chromatogram (TIC) is obtained, as shown in FIG. 7 (a). In this MS spectrum, peak f can be mentioned as a precursor ion candidate in accordance with the signal intensity criteria described above, but assuming that the mass-charge ratio corresponding to this peak f has been registered in the excluded ion list, it will not be selected as a precursor ion. Furthermore, peak g has a signal intensity below the threshold th, but assuming the corresponding mass-charge ratio has been registered in the priority ion list, the ion corresponding to this peak g will be automatically selected as a precursor ion, and MS2 analysis on this precursor ion will be performed immediately. As a result, the MS2 spectrum shown in B is obtained.
It will be assumed that the MS spectrum shown in C was obtained at another time t2. This MS spectrum has five peaks with a signal intensity at or above the threshold th, and the two peaks with the highest intensity are selected in sequence, but assuming the mass-charge ratios corresponding to peaks b and d have been registered in the excluded ion list, these will be excluded and the ions corresponding to peaks a and c of next highest intensity will be automatically selected as precursor ions, and MS2 analyses on these two precursor ions will be performed immediately. As a result, the two MS2 spectra shown under D and E are obtained. In analysis using an auto MSn function, conventional MS analysis not involving CID is repeatedly executed, and if there are ions which meet the precursor selection parameters based on the analysis results, they are set as precursor ions and MS2 analysis is performed in real time. It is possible to perform MSn analysis where n is 3 or greater by a similar method.
In a conventional chromatograph-mass spectrometer, when data collected using an auto MSn function as described above is analyzed and processed for display on the screen of a display unit, the MS spectrum obtained in the retention time designated by the analyst and the MS2 spectrum for the precursor ion automatically selected based on that MS spectrum are displayed next to each other on the same screen (see Non-patent literature 2). An example of mass spectrum display of this sort is shown in FIG. 8.
In FIG. 8, in the upper area 41 inside the mass spectrum display frame 40, the MS spectrum at retention time 12.05 (min) is displayed, and in the lower area 42, an MS2 spectrum using m/z 426 at the same retention time 12.05 (min) as a precursor ion is displayed. The m/z of the precursor ion is indicated by upward arrow 44 below the horizontal axis (m/z axis) of the MS spectrum in the upper area 41. This m/z 426 precursor ion was automatically selected from the MS spectrum displayed in the upper area 41, but there are multiple peaks in the MS spectrum having a signal intensity greater than the m/z 426 peak. For example, if the precursor ions were to be selected in order of signal intensity, ions corresponding to peaks of greater signal intensity should be selected, but that is not the case here, and the analyst has no way of knowing why the m/z 426 peak was selected as the precursor ion.
Of course, if the screen for setting the precursor selection parameters is open, one can view the excluded ion list, priority ion list, etc., as shown in FIG. 2, so the analyst can refer to that to check the mass-charge ratios of excluded ions and the mass-charge ratio of priority ions at the retention time being examined and the like. However, the manipulations and operations of newly opening and referring to this sort of other screen is troublesome and takes effort, and it is laborious to compare the numerical values listed in the excluded ion list and priority ion list to the MS spectrum and determine if a peak corresponds to an excluded ion or priority ion. In particular, as shown in FIG. 2, since setting of parameters with a high degree of freedom is possible, such as being able to arbitrarily set excluded ions and priority ions for each retention time range, the operation is complicated and not easy for the analyst to understand.
Furthermore, when performing MS3 analysis using an auto MSn function, if a peak appearing on the MS2 spectrum obtained through MS2 analysis using a specified ion as a precursor ion or using an automatically selected ion as a precursor ion is a product ion due to specified neutral loss, that ion may be used as a precursor ion for performing MS3 analysis. Namely, there are cases where producing a specified neutral loss is set as a precursor ion selection parameter for MSn analysis. When a precursor ion for MS3 analysis is discovered automatically under such precursor ion selection parameters and the data collected by performing MS3 analysis is analyzed and processed on a conventional chromatograph-mass spectrometer, three sets of analysis results are displayed next to each other on the same screen: the MS spectrum obtained at the retention time designated by the analyst, the MS2 spectrum for the precursor ion automatically selected based on said MS spectrum, and the MS3 spectrum for the precursor ion automatically selected based on the MS2 spectrum. In this case, if there are a large number of peaks appearing in the spectrum or if a peak with a relatively low intensity is a precursor ion, there is the problem that it is difficult for the analyst to intuitively understand which peak was selected as the precursor ion.